It is common in refrigerant units to utilize a mechanical throttling valve in the suction line to protect the prime mover, engine or motor, which drives a refrigerant compressor, from overload. The mechanical throttling valve limits the compressor suction pressure to a predetermined maximum value, to ensure that the compressor prime mover is not overloaded. U.S. Pat. No. 4,977,751, which is assigned to the same assignee as the present application, teaches the use of a controllable, proportional modulation valve in the suction line to prevent prime mover overload, eliminating the need for the mechanical suction line throttling valve. A proportional modulation valve is already present in many refrigeration units to provide more precise temperature control adjacent to a selected set point temperature SP.
Refrigerants which utilize a chlorofluorocarbon (CFC), such as refrigerants R12 and R502, are being phased out, and replaced by refrigerants which are more environmentally friendly, such as R404A. While the newer refrigerants have the advantage of not utilizing chemicals which are suspected of having a depleting effect on stratospheric ozone (O.sub.3), the newer refrigerants have different operating characteristics than the CFC refrigerants. For example, refrigerant R404A has a much steeper temperature-pressure curve than R502. As a result, we are experiencing shut downs on refrigeration units which utilize R404A during high ambient temperature conditions, due to a protective high pressure cut out switch (HPCO) functioning, notwithstanding controlling the suction line modulation valve to limit the load on the prime mover to a predetermined maximum value.
Thus, it would be desirable, and it is an object of the present invention, to reduce the incidence of refrigeration unit shutdowns due to high refrigerant discharge pressure.
When frozen loads, and other loads which do not require precise temperature control to prevent load damage, are being conditioned, it is common to select a start-stop cycling mode to control the temperature of a conditioned space to a predetermined band adjacent to a selected set point temperature, instead of utilizing a continuous operating mode. The start-stop cycling mode conserves fuel. However, it is possible during certain low ambient temperature conditions, such as when the ambient temperature is close to, or below, the cargo set point temperature SP for a frozen load, for the cycling rate to be excessive. An excessive rate would be that cycling rate where the mechanical wear and stress on system mechanical and electrical components due to stopping and starting offsets the fuel saving advantage.
Thus, it would be desirable, and it is another object of the invention, to prevent excessive cycling of a refrigeration unit set to operate in a cycle mode, during certain low ambient temperature conditions.